The present invention relates to a rubber composition for a tire and a run flat tire using the rubber composition for the reinforcing rubber layer of the sidewall.
At present, a run flat tire having a reinforcing rubber layer of high hardness positioned on the inside of the sidewall has been realized and such a tire can run for a certain distance even when air pressure is lost due to puncture of the tire. As a result, a spare tire does not always need to be carried and the weight of the vehicle as a whole can be reduced. However, the speed and distance that can be run by a run flat tire when punctured is limited and further improvement in durability of a run flat tire is desired.
An example of an effective means to improve durability of a run flat tire is the method of suppressing deformation by thickening the reinforcing rubber layer, thereby preventing damage due to deformation. However, because the weight of the tire increases, this method counters the initial purpose of a run flat tire, which is to become lightweight.
Another example of an effective means to improve durability of a run flat tire is the method of increasing the hardness of the reinforcing rubber by compounding an increased amount of a reinforcing filler such as carbon black, thereby suppressing deformation. However, improvement of run flat durability is not satisfactory, as the load to processes such as kneading and extruding is large and also, heat generating properties become high after vulcanization.
Furthermore, an example of a method for suppressing deformation and heat generation is the method of increasing vulcanization density by using a large amount of a vulcanizing agent and a vulcanization accelerator, without increasing the amount of carbon black (for example, see JP-A-2002-155169). However, elongation and strength at break of the rubber decrease.